In electrostatographic reproduction apparatus, an electrostatic latent image is formed on a primary image-forming member such as a photoconductive surface and is developed with thermoplastic toner particles to form a toner image. The toner image is thereafter transferred to a receiver member, e.g., a sheet of paper or plastic, and the toner image is subsequently fused or fixed to the receiver member in a fusing station using heat and pressure. The fusing station includes a fuser member which can be a roller, belt, or any surface having a suitable shape for fixing thermoplastic toner particles to the receiver member.
In fusing using a roller fuser member, the toned receiver member is commonly passed between a pair of engaged rollers that produce an area of pressure contact known as a fusing nip. In order to form the fusing nip, at least one of the rollers typically includes a compliant or conformable layer. Heat is transferred from at least one of the rollers to the toner in the fusing nip, causing the toner to partially melt and attach to the receiver member. In the case where the fuser member is a deformable heated roller, a resilient elastomeric layer is typically bonded to the core of the roller, with the roller having a smooth outer surface. Where the fuser member is in the form of a belt, e.g., a flexible endless belt that passes around the heated roller, the belt typically has a smooth outer surface which may also be hardened.
Simplex fusing stations attach toner to only one side of the receiver member at a time. In this type of fusing station, the engaged roller that contacts the unfused toner is commonly known as the fuser roller and is a heated roller. The roller that contacts the other side of the receiver member is known as the pressure roller and is usually unheated. Either or both rollers can have a compliant layer on or near the surface. It is common for one of these rollers to be driven rotatable by an external source while the other roller is rotated frictionally by the nip engagement.
A conventional toner fuser roller commonly includes a rigid cylindrical core member, typically a metallic core such as aluminum, coated with one or more synthetic layers usually formulated with polymeric materials made from elastomers. A resilient base cushion layer, which may contain filler particles to improve mechanical strength and/or thermal conductivity, is typically formed on the surface of the core, which may advantageously be coated with a primer to improve adhesion of the resilient layer. Roller base cushion layers are commonly made of silicone rubbers or silicone polymers such as, for example, polydimethylsiloxane (PDMS) polymers disclosed by the Chen, et al. patents (U.S. Pat. Nos. 5,960,245 or 6,020,038).
The most common type of fuser roller is internally heated, i.e., a source of heat is provided within the roller for fusing. Such a fuser roller generally has a hollow core, inside of which is located a source of heat, usually a lamp. Less common is an externally heated fuser roller, which fuser roller is typically heated by surface contact with one or more heating rollers. Externally heated fuser rollers are disclosed by the O'Leary patent (U.S. Pat. No. 5,450,183), the Derimiggio, et al. patent (U.S. Pat. No. 4,984,027), the Aslam, et al. patent (U.S. Pat. No. 6,567,641), and the Chen, et al. patent (U.S. Pat. No. 6,490,430).
Some roller fusers rely on film splitting of low viscosity oil to enable release of the toner and (hence) receiver member from the fuser roller. The oil is typically applied to the surface of the fuser from a donor roller coated with the oil provided from a supply sump. A donor roller is disclosed in the Chen, et al. patent (U.S. Pat. No. 6,190,771) and in the Chen, et al. patent application (U.S. patent application Ser. No. 09/960,661, filed Sep. 21, 2001).
Release oils (commonly referred to as fuser oils) are composed of, for example, polydimethylsiloxanes. When applied to the fuser roller surface to prevent the toner from adhering to the roller, fuser oils may, upon repeated use, interact with PDMS material included in the resilient layer(s) in the fuser roller, which in time can cause swelling, softening, and degradation of the roller. To prevent these deleterious effects caused by release oil, a thin barrier layer made of, for example, a cured fluoroelastomer and/or a silicone elastomer, is typically formed on the resilient cushion layer, as disclosed in the Davis, et al. patent (U.S. Pat. No. 6,225,409).
In the fusing of the toner image to the receiver member, the area of contact of a conformable fuser roller with the toner-bearing surface of a receiver member sheet as it passes through the fusing nip is determined by the amount of pressure exerted by the pressure roller and by the characteristics of the resilient cushion layer. The extent of the contact area helps establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser roller.
Prior art internally heated fuser rollers typically have one or more synthetic polymeric layers including a deformable layer such as a base cushion layer surrounding a hollow metallic core member, with a source of heat such as a lamp provided within the hollow core member. Such fuser rollers rely on thermal conductivity through the synthetic layers for conduction of heat from the source of heat to the surface of the roller so as to provide heat for fusing toner particles to receiver members. The thermal conductivity, attainable by the use of one or more suitable particulate fillers, is determined by the filler concentration. The thermal conductivity of most polymers is very low and the thermal conductivity generally increases as the filler concentration is increased. However, if the filler concentration is too high, the mechanical properties of a polymer are usually compromised. For example, the stiffness of the synthetic layers may be increased by too much filler so that there is insufficient deformability to create a wide enough nip for proper fusing. Moreover, too much filler will cause the synthetic layers to have a propensity to delaminate or crack or otherwise cause failure of the roller.
Because the mechanical requirements of such an internally heated fuser roller require that the filler concentrations be moderate, the ability of the roller to transport heat is thereby limited. In fact, the concentration of filler in prior art internally heated deformable fuser rollers has reached a practical maximum. As a result, the number of copies that can be fused per minute is limited, and this in turn can be the limiting factor in determining the maximum throughput rate achievable in an electrostatographic printer. There is a need, therefore, to provide an improved fusing roller for increasing the number of prints that can be fused per minute, thereby providing opportunity for higher machine productivity.
An auxiliary internal source of heat may optionally be used with an externally heated fuser roller, e.g., as disclosed in the Stack, et al. patent (U.S. Pat. No. 6,567,641) and in the Chen, et al. patent (U.S. Pat. No. 6,490,430). Such an internal source of heat is known to be useful when the fusing station is quiescent and/or during startup when relatively cold toned receiver members first arrive at the fusing station for fusing therein. It will be evident from the preceding paragraph above that in order for such an auxiliary internal source of heat to be effective (when intermittently needed), the fuser roller must have a sufficiently large thermal conductivity. However, this requirement conflicts with a need to keep heat at the surface of an externally heated fuser roller, i.e., so as not to unnecessarily conduct heat into the interior which would compromise the fusing efficiency of the roller. Thus there remains a need to provide an improved efficiency fusing roller so that the throughput rate of an electrostatographic printer can be increased over that of prior art.